HDI PCB technology has played a pivotal role in driving progress within the electronics industry, facilitating the creation of densely interconnected and intricately designed circuitry. This technology has been instrumental in pushing the limits of miniaturization and enhancing functionality across various sectors, ranging from wearable consumer devices to industrial, military, and aerospace systems. The adoption of HDI PCBs has paved the way for advanced heterogeneous 2.5D and 3D semiconductor packages, contributing to the development of smaller, faster, and more functional devices that are widely enjoyed today.
Despite these advancements, the production and assembly of HDI PCBs present distinct challenges. Achieving high-precision conductive patterns and traces demands specialized manufacturing capabilities, especially when dealing with high PCB layer counts (exceeding 10 layers).
In a recent project at PCB Trace Technologies, we successfully manufactured a 20-layer HDI PCB for a commercial aerospace company. To enhance the conductive circuit patterns, we employed a modified semi-additive process (mSAP) as an alternative to subtractive etching. This approach allowed for superior pattern geometry, enabling the creation of finer traces with tighter spacing, reaching as narrow as 3 mil (0.075 mm).
Leveraging our expertise in HDI technology, we aim to share key best practices to assist others in the industry in navigating the complexities of designing and fabricating HDI circuitry.
HDI Guidelines and Standards
Several IPC standards provide comprehensive guidance on HDI PCB design and manufacturing, offering advantages such as improved efficiencies, process controls, enhanced product quality, and increased customer satisfaction:
IPC-2315: HDI Design Guideline outlines design recommendations for HDI PCBs.
IPC-2226: An extension of generic guidelines for PCB design (IPC-2221), providing in-depth standards and guidance for HDI PCB design.
IPC-4104: Specifies appropriate material sets for HDI PCBs, covering various conductive and dielectric materials.
IPC-6016: A design qualification standard for HDI PCBs.
Design Considerations for HDI Circuits
Successful design and fabrication of HDI PCBs require careful consideration of several factors:
Meticulous planning for component layout and placement to optimize board real estate and prevent signal interference.
Monitoring minimum trace widths and spacing, which can be incredibly narrow, often reaching 2 mils (0.002 inches) or smaller.
Attention to impedance control, crucial for handling high-speed signals, ensuring proper matching and controlled trace geometries.
Consideration of thermal management strategies, including the use of thermal vias and copper pours, to prevent overheating and ensure long-term reliability.
Selection of materials based on characteristics such as dielectric constant, thermal properties, and mechanical strength.
Vias in HDI PCBs
HDI PCBs utilize various types of vias for vertical electrical connections, each serving a unique purpose and influencing routing density and signal integrity:
Through-hole vias (THVs): Traverse the entire thickness of the board, suitable for most applications, including high-power designs.
Blind vias: Connect outer layers to one or more inner layers without traversing the entire PCB.
Buried vias: Link inner layers without extending to the board's surface.
Microvias: Used in dense, high-layer count boards with diameters smaller than 6 mils.
Via-in-pad: Places vias directly under surface-mount component solder pads, requiring filling and capping to prevent solder wicking.
Via Plating
Traditionally, electroless copper plating has been the preferred method for plating via holes in PCBs. However, innovative alternatives, such as direct metallization, offer shorter cycle times, improved hole wall reliability, and significant reductions in environmental impact and costs compared to electroless copper plating.
Future of HDI
The global HDI PCB market, valued at $8.2 billion in 2022, is anticipated to reach $11.3 billion by 2028. This growth presents a promising opportunity for PCB fabricators. Embracing the trend towards smaller, sleeker, and more functional devices positions industry participants as agile leaders in HDI technology, capable of assisting customers in pushing the boundaries of miniaturization in next-generation electronics.